The present invention generally relates to firearms, and more particularly to breech bolts for firearms.
Bolt action rifles comprise a manually retractable and rotatable bolt used to form a closed locked breech. The front end of the bolt is equipped with bolt lugs which rotatably engage mating locking lugs disposed at the rear of the barrel to form a locked breech for discharging the rifle and prevent escape of combustion gases. A bolt handle coupled to the bolt allows a user to rotate the bolt between locked and unlocked rotational positions, and to advance/retract the bolt between axial closed and open breech positions for loading/unloading cartridges from the breech chamber formed in the rear end of the barrel. Bolt action rifles typically feed cartridges from a single stack magazine. Accordingly, the ability to fully rotate the bolt between locked breech and unlocked breech positions while the bolt is in battery with the chamber is important.
Bolt action rifles typically have bolts with two or three locking lugs, although some designs may have more. The bolt lugs form the locked breech by engaging the corresponding locking lugs in the receiver or barrel at the rear of the barrel chamber once the bolt is manually rotated to overlap the mating locking surfaces (the bolt lugs being positioned in front of the locking lugs). Bolts with two lugs typically operate with the lugs in the horizontal or 3 o'clock and 9 o'clock position when the breech is locked, and rotate 90 degrees into the 6 and 12 o'clock positions when feeding cartridges into the breech. The bolt lug at the 6 'oclock position strips a fresh cartridge from the magazine and chambers the round. This geometry, however, is not conducive to feeding cartridges from double stack magazines, like used in the AR-15 style rifles as one example, because the round body of the bolt does not allow the lower front face of the bolt lug to extend far enough down in between the magazine feed lips necessary to reliably strip a cartridge from a double stack magazine when the action is cycled.
Bolts with three lugs typically feed cartridges with one of the lugs at the 6 o'clock position from a single stack magazine. One advantage of three lugs is that the bolt need not be rotated a full 90 degrees to lock and unlock the breech, thereby making it easier and less cumbersome for the user. These bolts often have bolt bodies substantially similar in diameter to the bolt lugs, and thus also do not allow enough cartridge contact to feed from double stack magazines like used in the AR-15 style rifles. In addition, these full diameter bolt bodies do not fit between the magazine feed lips thereby preventing the bolts from advancing far enough forward to strip a cartridge from the magazine in the first instance. The bolt body immediate behind the front bolt lugs would contact the rear of the magazine feed lips, preventing full forward motion of the bolt to close the breech.
Bolt designs used for AR-15 style rifles with double stack magazines do not provide a solution for the cartridge feed problem associated with manually rotated bolts used in bolt action rifles. In contrast to conventional one-piece bolts used in bolt action rifles, a significant difference is that AR-15 bolt assemblies have a two-piece construction comprised of an outer non-rotatable bolt housing (often called bolt carrier) that carries a rotatable bolt therein. Only the head of the bolt with exposed bolt lugs typically protrudes from the front end of the housing for lockup with the firearm's locking lugs to lock the breech. A camming mechanism automatically rotates the bolt independently of and in relation to the non-rotatable housing when the bolt housing is moved into or out of engagement with the locking lugs to lock or unlock the breech respectively.
Although the AR-15 bolt housings may sometimes have narrow longitudinal slots formed in the lower half of the bolt housing to avoid interference with the feed lips of double stack magazines, this design is not readily adaptable for use with one-piece solid bolt action rifle bolts because the bolt housing does not need to rotate when positioned over the magazine feed lips to lock the breech due to the independently rotating bolt.
An improved rotatable bolt design is desired that allows AR-15 double stack ammunition magazines to be used with bolt action rifles having three-lug bolts.